First thing I would check is that the heatsink is making good contact with the CPU. I had this exact same problem, and I tried a different video card, RAM, PSU, HDD...basically everything under the sun.
It turned out that when I installed my new heatsinks 2 weeks earlier, I had forgotten to clean off the sticky stuff left behind by the clear protective film on the bottom of the 'sink. I cleaned the 'sinks and CPU's with some Isopropyl Alcohol, re-applied some Arctic Silver and replaced the 'sinks. Problem solved.
Worth a shot if nothing else works.
